1. Field of the Invention
The present invention relates generally to tunnel excavating methods, and more particularly to a tunnel blasting method using an air bladder, in which one or more air bladders are situated in each blast hole to form a front free surface in each cut hole or one or more small free surfaces in the other blast holes, thereby decreasing blast vibration and blast noise, reducing the amount of explosive considerably, preventing the production of boulders and suppressing initial vibration.
2. Description of the Prior Art
As well known to those skilled in the art, tunnel excavating methods using blasting techniques may be classified into a full-scale blasting method, a divisional blasting method and a multistage blasting method. The tunnel excavating methods using blasting techniques are performed with the following three stages in common.
A first stage is the stage of drilling cut holes, cut spreader holes, floor holes and roof holes to predetermined depths, a second stage is the stage of loading the drilled holes with detonators and explosives, and a third stage is the stage of detonating the detonators using a triggering device.
As shown in FIGS. 1 to 2d, in the conventional blasting methods, positions where cut holes 2a and 2b, cut spreader holes 2c, floor holes 2d and roof holes 2e are to be drilled are marked on a working face, the blast holes are drilled to appropriate depths on the working face at the marked positions using a drilling machine such as a rock drill or jumbo drill, and the blast holes are respectively loaded with delay detonators and explosives.
With regard to the above-described tunnel blasting stages, a detailed description of the drilling and detonating stage are not necessary because these stages are well known, but the hole loading stage should be described in detail because this stage is complicated.
FIGS. 2a to 2d are views, which are concerned with prior art and show states in which the blast holes are respectively loaded with delay detonator and explosive combinations and are respectively stemmed with stemming material 9. In the drawings, FIGS. 2a and 2b are views respectively showing states in which the cut holes 2a and 2b are loaded with the delay detonators and the explosives and stemmed with stemming material 9. FIG. 2c is a view showing a state in which the cut holes 2a and 2b are loaded with the delay detonators and the explosives and stemmed with stemming material 9.
In most blast hole loading stages of the conventional tunnel blasting methods, explosives are detonated in a state in which the lower portions of blast holes are loaded with delay detonators 4 and explosives 5 and the upper portions of the blast holes are loaded with stemming materials 9.
In such cases, the blast holes are loaded with the delay detonators 4 and the explosives 5 at their lower portions. The explosives 5 employed in these cases may be classified into general explosives and precision explosives. The cut holes 2a and 2b, the cut spreader holes 2c and the floor holes 2d are loaded with the general explosives 5, while the roof holes 2e and other outer side holes are loaded with the general explosives and the precision explosives so as to reduce the damage to the parent rock. In some cases, the roof holes are alternatively loaded with the explosives 5 so as to form non-loaded blast holes 3.
The cut holes 2a and 2b are loaded with delay detonators in a way of symmetrically situating the detonators in the cut holes 2a and 2b in the upward order. The cut spreader holes 2c, the floor holes 2d and the roof holes 2e are loaded with delay detonators in a way of situating the detonators in the holes 2c, 2d and 2e in the order of progressing from the center to the outside. In such a state, the tunnel is blasted by detonating the detonators using a triggering device. The detonations of the detonators are sequentially performed in the order of the cut holes 2a and 2b, the cut spreader holes 2c, the floor holes 2d and the roof holes 2e. 
Referring to FIGS. 3a and 3c, in the conventional tunnel blasting method, since the explosion length of the explosive 5 is confined to the length of the explosive 5 and the length l1 of the stemming material 9 is relatively large in comparison with the depth of the blast hole 2, the projective area of the explosives 5 can be enlarged when the explosives 5 are detonated in the blast holes 2. Therefore, in the case of the conventional tunnel blasting method, since the explosion power of the explosives 5 is not fully utilized to fragment a rock, it is not an effective tunnel blasting method.
The detonators 4 and the explosives 5 are situated in the center and inner portions of the blast holes 2, so that the vibration of the ground is great. Additionally, upon detonation of the explosives, the stone formation portion A of a rock inside a fracture boundary line L is fragmented into a large number of stones, while the boulder formation portions B of the rock outside the fracture boundary line L are fractured into a small number of boulders. The explosion power of the explosives cannot reach the boulder formation portions B around the stemming materials 9 due to the presence of the stemming materials 9, so that the boulder formation portions B are not fragmented into the stones.
Meanwhile, although the roof holes 2e are loaded with the precision explosives 6 so as to smooth the blasted surfaces of a roof and sidewalls, there occur problems in which the precision explosives are expensive, the usage of the precision explosives is difficult and the safety of work is deteriorated due to blast failure.
The conventional tunnel blasting method utilizes a single free surface. Consequently, unacceptable blast vibration and blast noise are generated because the explosive charges are concentrated in the inner portions of the blast holes, and an additional post-process such as a shotcreting process is required because the wall surface of the blasted tunnel is uneven.
In addition, since the tunnel blasting is performed using a single free surface, the travel distance of the fly rocks is increased, thereby requiring a long safety distance.
In particular, in the tunnel blasting methods, the vibrations of the ground generated upon blasting displease persons and damage neighboring structures, so that complaints against the operations may be made to hinder the operations. Accordingly, the development of a technique for reducing blast vibrations in the cut holes that greatly influences the vibration of the ground is urgently required.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a tunnel blasting method, in which cut holes are drilled deeper than the other blast holes and air bladders are situated in the extended portions of the cut holes to form front free surfaces, thereby suppressing initial blast vibration and blast noise generated in the cut holes and increasing the advances of the cut holes.
Another object of the present invention is to provide a tunnel blasting method, which increases a specific surface area on which explosion power is exerted, provides an additional free surface and allows explosives to be situated near a working face, thereby reducing the total amount of explosives used.
A further object of the present invention is to provide a tunnel blasting method, in which explosives are detonated at predetermined time intervals using the action of sympathetic blasting so as to reduce blast vibration and rubber plugs are inserted into the outer portions of blast holes to reduce blast vibration and blast noise.
Yet another object of the present invention is to provide a tunnel blasting method using an air bladder, which allows blast holes to be loaded with cartridge explosives and causes the explosive energy of explosives to be uniformly exerted on the surfaces of the blast holes, thereby blasting a tunnel at a low cost and obtaining a smooth blasted surface.
In order to accomplish the above object, the present invention provides a tunnel blasting method, comprising the steps of drilling blast holes such as cut holes, cut spreader holes, floor holes and roof holes to predetermined depths and in a predetermined hole arrangement, loading the blast holes with one or more detonators and explosives, stemming the blast holes with stemming materials and detonating the detonators using a triggering device, wherein one or more air bladders are situated in each of the blast holes so that a front free surface or one or more small free surfaces are formed, thereby enlarging a projective area toward a free surface and increasing a total blast pressure so as to increase the fragmentation rate of a rock and reduce blast vibration.
In accordance with a feature of the present invention, the cut holes are drilled deeper than an advance line and air bladders are inserted into the deeper portions of the cut holes, so that front free surfaces are respectively formed in the cut holes and explosives are situated near a free surface, thereby distributing blast vibration toward the front free surfaces and the free surface.
In accordance with a feature of the present invention, each of the blast holes is alternately loaded with the explosives and the air bladders.
In accordance with a feature of the present invention, a single detonator is situated in each blast hole so that the explosives are detonated by the action of sympathetic detonation.
In accordance with a feature of the present invention, each of the blast holes is sealed at its outer portion with a rubber plug so as to reduce blast noise.
In accordance with a feature of the present invention, the diameter of each of the air bladders is equal to or less than the diameter of each of the blast holes.
In accordance with a feature of the present invention, the air bladders are made of synthetic resin, such as polyethylene, polypropylene, polyester or polyamide.